The present invention relates to an optical information recording medium and an optical information reproducing device for optically reproducing information recorded on the optical information recording medium such as an optical disk.
In recent years, much research and development has been actively pursued in the area of recording and reproducing techniques with higher density. For example, a so-called land and groove recording system is proposed in which information is recorded to both a land section and a groove section of a recording medium surface to increase the capacity. One technique for reproducing information recorded onto the optical disk by the land and groove recording system, is disclosed in the article "High Track Density Magneto-Optical Recording Using a Crosstalk Canceler", by K. kayanuma et al., SPIE, Vol. 1316, pp 35-39, 1990.
The above technique will be explained with reference to FIGS. 1 and 2. Specifically, as shown in FIG. 1, three adjacent tracks on an optical disk are irradiated with three light beams 21, 22, and 23, which are shown by circles having slanting lines. In this case, three adjacent tracks are a land section track 11 of an inner peripheral side, a central groove section track 12, and a land section track 13 of an outer peripheral side.
Then, as shown in FIG. 2, reproducing signals 31, 32, and 33 due to three light beams 21, 22, and 23 are input to a timing control section 51 to perform a phase adjustment. Then, reproducing signals 34, 35, and 36 due to light beams 21 and 23 of both sides among phase-adjusted reproducing signals 34, 35, and 36 are gain-controlled by a variable gain control section 52 to obtain signals 37 and 39. Thereafter, the gain-controlled signals 37 and 39 are added to produce signal 40, which is subtracted from a reproducing signal due to the central light beam 22 by a subtraction processing section 53. Thereby, there can be finally obtained a reproducing signal 41 in which crosstalk due to a recording mark 10 shown by a white circle of FIG. 1 is reduced. In this case, the gain of the variable gain control section 52 is set to maximize quality of the reproducing signal 41.
According to the above-explained prior art, the crosstalk from both adjacent tracks is subtracted from the reproducing signal due to the central light beam by an electrical processing in the land and groove recording system. As a result, the recording capacity can be increased as twice that of the traditional optical disk in which the recording mark is formed in only the groove section or the land section.
Thus, according to the above-explained prior art, the electrical processing including the complicated phase adjustment against the reproducing signal and the complicated gain adjustment are needed to reduce the crosstalk. As a result, the adjustment error in the phase and the gain occurs in accordance with increase in the recording density, that is, track density. Moreover, a crosstalk reduction effect is decreased by a difference of the adjustment caused by a change in the environment and imperfections inherent with the passage of time. In other words, the optical disk on which information is recorded with high density by the land and groove recording system is irradiated with three light beams to obtain the reproducing signal. The obtained reproducing signal is electrically processed to reduced the crosstalk, so that the recording density can be improved. As a result of the needed complicated adjustments, if the recording density is increased, there occurs a problem in that the crosstalk reduction effect is deteriorated by the adjustment error and the difference of adjustment.
Moreover, it is assumed that the above-mentioned prior art is used in a ZCAV (Zoned Constant Angular Velocity) recording system in which linear recording density is constantly maintained over the entire surface of the disk in order to obtain a large capacity. In this case, since a signal frequency differs every radial position of the disk, the phase adjustment due to the timing control section 51 becomes more complicated. As a result, the adjustment error and the adjustment difference in the phase and the gain more easily occur, and this brings about the deterioration of the crosstalk reduction effect.
To solve the above problem, as described in U.S. Pat. No. 5,168,490, there is disclosed a recording medium in which one of adjacent tracks has an area with a first phase depth (about .pi. radians or geometrical depth of about .lambda./4) and the other has an area with a second phase depth (about 2 .lambda./3 radians or geometrical depth of about .lambda./8). In this case, .lambda. is a wavelength of a reading beam.
In a reproducing information recorded on device for reproducing a recording medium in which the phase depth is changed to d1=.pi., d2=2 .pi./3 every one track, as specifically explained later, signals are detected by two detectors. Then, a sum signal of detected signals obtained by two detectors is used as an information reproducing signal from the track of the first phase depth. Moreover, a differential signal of detected signals obtained by two detectors is used as an information reproducing signal from the track of the second phase depth.
However, in the track of the second phase depth for reproducing the differential signal, if the recording density is increased, the peak position of the differential signal corresponding to the pit edge position varies, and high density can not obtained.